One of the two pilots on a doomed Germanwings flight was locked out of the cockpit before the plane crashed in the French Alps, killing 150 people, a source close to the investigation said on Thursday.

Cockpit recordings recovered from the crash site indicated one of the seats was pushed back and the door opened and closed, followed by the sound of knocking, said the source, adding “there was no more conversation from that point until the crash”. The source said an alarm indicating the proximity of the ground could be heard before the impact.

The recording showed the pilots speaking normally and in German at the start of Flight 4U9525 early on Tuesday. The source could not say if it was the captain or the first officer who had left the cockpit.

The cockpit voice recorder was found late Tuesday, several hours after the crash of the budget flight from Barcelona to Duesseldorf, and the data analysed on Wednesday afternoon.

The Airbus A320 suddenly began a fatal eight-minute descent shortly after reaching cruising altitude. No distress signal was sent and the crew failed to respond to desperate attempts at contact from ground control.